Color-plus-clear coating systems involving the application of a colored or pigmented basecoat to a substrate followed by application of a transparent or clearcoat over the basecoat have become increasingly popular as original finishes for a number of consumer products including, for example, automotive vehicles. The color-plus-clear coating systems have outstanding appearance properties such as gloss and distinctness of image, due in large part to the clearcoat. Such color-plus-clear coating systems have become popular for use with automotive vehicles, aerospace applications, floor coverings such as ceramic tiles and wood flooring, packaging coatings and the like.
A typical automotive coating system can include the sequential application of an electrodeposition primer, a primer-surfacer, a color providing base coat, and a transparent topcoat. Also, adhesive coatings or layers, for example, windshield adhesives, trim and molding adhesives and structural adhesives are sometimes applied to the cured topcoats. In fabricating these multi-layer composite coatings, it is necessary that the various layers have acceptable interlayer adhesion.
On commercial automobile coating lines during application of the coating system, certain portions of the line can experience occasional process problems, for example, where the color coat applicator malfunctions, or curing oven faults where temperatures are out of specification. In these instances where the clear coat application system is malfunctioning, some automobile manufacturers may elect to fully cure the applied color coat and then reapply the color coat over the fully cured color coat prior to application of the clearcoat. In such situations, the fully cured color coat can have poor intercoat adhesion with the subsequently applied color coat, even though the compositions may be the same.
Further, during the assembly process, the applied color-plus-clear coating can include surface defects in the clear coat surface that requires repair. Some automobile manufacturers may elect to remove the defect and recoat the repair area with the same clear coat composition. In this instance, the cured clear coat must have excellent intercoat adhesion to the subsequently applied clear coat. It is known, however, that some clear coats when cured have poor intercoat adhesion with the subsequently applied repair clear coat.
Also, windshields and other items such as trim moldings typically are affixed to the body of a vehicle with an adhesive material, typically a moisture-cured material containing isocyanate group-containing polymers. Motor Vehicle Safety Standards (MVSS) require that these adhesives have complete adhesion to both the windshield and the coated substrate to which they are applied. These adhesive products adhere well to many cured topcoating compositions used to coat vehicles such as automobiles. It is known, however, that these adhesive materials often do not completely adhere to some topcoats, for example, those formed from coating compositions based on carbamate and/or urea containing polymers. This necessitates the application of a primer coating to the cured carbamate and/or urea-based top coatings prior to application of the windshield adhesive to ensure compliance with the aforementioned Motor Vehicle Safety Standards. Use of such primer coatings has proven to be effective, but primer coating application adds an additional and expensive step to the windshield and/or trim installation processes.
In view of the foregoing, there is a need in the coating industry for coating compositions that have improved intercoat or interlayer adhesion properties.
It has now been found that certain adhesion-promoting agents that have surface active properties such that the solubility parameter of the coating composition containing the adhesion-promoting agent is sufficiently different from that of an analogous coating composition which does not contain the adhesion-promoting agent; the adhesion-promoting agent partitions to the surface region of the resultant coating. This can result in a concentration of the adhesion-promoting agent at the surface region which is greater than the concentration in the interior or bulk region of the coating layer. This partitioning effect of the adhesion-promoting agent can significantly increase its effect in promoting the adhesion of the coating layer that contains the adhesion-promoting agent to a subsequently applied coating layer, as well as to the substrate to which it is applied.